1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile and the like, and more particularly, it relates to an image forming apparatus in which an image on an image bearing member is transferred onto an intermediate transfer member and then the image on the intermediate transfer member is transferred onto a transfer material.
2. Related Background Art
FIG. 9 shows an example of a conventional color image forming apparatus. In FIG. 9, a photosensitive drum (image bearing member) 104 is rotated by driving means (not shown) in a direction shown by the arrow and is uniformly charged by a primary charger 105. Then, a laser beam corresponding to a magenta image pattern is illuminated onto the photosensitive drum 104 from an exposure device 110 to form a latent image on the photosensitive drum 104. When the photosensitive drum 104 is further rotated in the direction shown by the arrow, among developing devices 107a, 107b, 107c, 107d supported by a rotary supporting member 107e, a magenta developing device 107a containing magenta toner is rotated to be opposed to the photosensitive drum 104, so that the latent image is visualized by the magenta developing device 107a as a toner image.
An intermediate transfer belt (intermediate transfer member) 103 is passed over on three rollers 121, 122, 123 and is rotated at a speed substantially the same as that of the photosensitive drum 104 in a direction shown by the arrow. By impressing primary transfer bias to a primary transfer roller 102, the toner image formed on the photosensitive drum 104 is primary-transferred onto an outer peripheral surface of the intermediate transfer belt 103.
By repeating the above-mentioned process with respect to a cyan color, an yellow color and a black color by using the developing devices 107b, 107c, 107d, respectively, plural color toner images are formed on the intermediate transfer belt 103.
Then, a transfer material 113 is fed from a transfer material cassette 113a by a pick-up roller 112 at a predetermined timing. At the same time, secondary transfer bias is impressed to a secondary transfer roller 108, with the result that the toner images on the intermediate transfer belt 103 are transferred onto the transfer material 113 collectively. Then, the transfer material 113 is conveyed, by a conveying belt 120, to a fixing device 109, where the toner images are fused and fixed, thereby obtaining a color image.
After the secondary transfer, residual toner remaining on the intermediate transfer belt 103 is charged with opposite polarity by a charging device 101a as an intermediate transfer belt cleaning device 101, with the result that the residual toner is counter-transferred onto the photosensitive drum 104, thereby cleaning or removing the residual toner. On the other hand, residual toner after primary transfer and counter-transfer toner on the photosensitive drum 104 are removed by a photosensitive drum cleaning device 106.
Now, the cleaning of the intermediate transfer belt will be fully described.
Residual toner after secondary transfer is subjected to a strong electric field having polarity opposite to that of the toner when the toner is transferred from the intermediate transfer belt 103 to the transfer material 113, with the result that the residual toner may remain on the intermediate transfer belt 103 in a condition that the residual toner has been charged with polarity (positive polarity in this conventional case) opposite to normal charging polarity (negative polarity in this conventional case) of the toner. However, all toners cannot be reversed to the positive polarity, but there are toners partially neutralized not to have charges and/or toners remaining in negative polarity.
Thus, the charging device 101a is disposed at a downstream side of a secondary transfer position, i.e., immediately behind the secondary transfer position in a moving direction of the intermediate transfer belt 103 so that bias obtained by superposing an AC component with a DC component is impressed as a intermediate transfer belt cleaning bias by the charging device 101a. The residual toner after secondary transfer is reciprocally moved by the AC component repeatedly so that the toner is charged more uniformly with positive polarity.
The uniformly and positively charged residual toner after secondary transfer is counter-transferred onto the photosensitive drum 104 at a primary transfer nip portion and then is collected by the cleaning device 106 from the photosensitive drum 104.
In a print operation in which a plurality of transfer materials are continuously printed, the charges of the oppositely charged residual toner after secondary transfer on the intermediate transfer belt 103 and the charges of the normal toner (to be primary-transferred) on the photosensitive drum 104 do not cancel each other by short time contact therebetween. Thus, at the primary transfer nip portion between the photosensitive drum 104 and the intermediate transfer belt 103, when the primary transfer bias is impressed, the residual toner after secondary transfer and the toner on the photosensitive drum 104 can be substantially simultaneously transferred onto the photosensitive drum 104 and the intermediate transfer belt 103, respectively. Accordingly, since the residual toner after secondary transfer is not transferred onto the transfer material 113 in the next print, a proper image can be outputted.
When the above-mentioned intermediate transfer belt cleaning device 101 is used, a waste toner container for collecting the residual toner on the intermediate transfer belt 103 can also serve as the photosensitive drum cleaning device 106 so that the apparatus can be made compact and a maintenance ability can be improved.
By the way, in color image forming apparatuses such as the above-mentioned conventional color image forming apparatus, recently, an automatic both-side printing function has frequently been added. As shown in FIG. 10, the automatic both-side printing function is realized by optionally adding a both-side unit 130 to the conventional image forming apparatus externally or internally.
In the image forming apparatus having the automatic both-side printing function, there is provided selecting means 111 for selecting between a conveying path for discharging a sheet and a conveying path communicating with the both-side unit 130, in dependence upon whether the transfer material 113 discharged from the fixing device 109 relates to a first side print or a second side print. Further, the transfer material 113 conveyed to the both-side unit 130 by the selecting means 111 remains within the both-side unit 130 until an image to be printed in the second-side print is prepared on the photosensitive drum 104, and is re-fed in sync with a second-side image formation.
Several (1 to 3) transfer materials 113 can be stocked within the both-side unit 130 and the transfer material 113 is re-fed in response to a signal from control means (not shown) capable of judging a property of the image (whether a first-side image or a second-side image).
In the both-side print as mentioned above, since electric properties (resistance value, change in electrostatic capacity due to partial adhesion of toner layer and the like) and mechanical properties (rigidity of transfer material, curling condition and the like) of the transfer material to which the first-side image was transferred and fixed are changed, a second-side transferring condition will differ from the first-side transferring condition greatly. As a result, second-side image quality may considerably be worsened in comparison with first-side image quality.
To improve this, there has been proposed a technique in which the second-side image quality is improved by changing the transfer bias in the second-side transferring.
However, in the image forming apparatus using the intermediate transfer member cleaning device 101 as mentioned in connection with the above conventional technique, when the cleaning of the intermediate transfer belt is effected, the cleaning permitting conditions are that the residual toner after secondary transfer remains on the intermediate transfer belt 103 as little as possible and almost all of the toner has positive polarity. Therefore, even when the second-side transferring ability is enhanced, if an amount of the residual toner after secondary transfer having negative polarity is great, the residual toner after secondary transfer cannot be charged uniformly by the charging device 101a, with the result that the subsequent counter-transferring process at the primary transfer position becomes incomplete, thereby affecting a bad influence of intermediate belt cleaning failure upon the next image.
Particularly, in case of full-color image formation, even for the first-side print, the transferring condition for uniformly secondary-transferring all of the components from a monochromatic half-tone image to four color superposed images without poor image due to abnormal electric discharging is very severe, and, in the second-side print in which the transferring condition becomes more severe, an area where the residual toner after secondary transfer having negative polarity exists is generated on the same image without fail, thereby causing the intermediate transfer belt cleaning failure.